Prior art wavelength selective devices are constructed with discrete structures coupled through free space. This type of assembly suffers a number of limitations. Optical alignment of the elements is highly sensitive and costly to produce. In the interests of reliability and robustness to environmental factors, it is desirable to perform as many of the required functions as possible on a monolithically integrated planar lightwave circuit. (PLC).
Planar lightwave circuits (PLC) are constructed as lithographically formed waveguides on a planar substrate. The waveguides are surrounded by a lower index cladding material for confining optical signal within the waveguide circuit. PLCs are frequently constructed as silica on silicon assemblies in which a silicon substrate is deposited with a silica lower cladding, waveguide cores of germanium doped silica are formed and an upper cladding of borophosphosilicate glass is deposited over the waveguide cores. Alternatively, PLCs are formed in InGaAsP or optically transmissive polymer or glass. Structures formed in the PCL include channel waveguides which confine the signal in two dimensions orthogonal to the direction of light propagation, and slab regions which confine the optical signal in one dimension and allow a wavefront to spread over a defined region in the orthogonal dimension. In optical communications, the dimensions are typically confined to single mode transmission.
A typical demultiplexer for separating the multiplexed optical beam is an arrayed waveguide diffraction grating (AWG) constructed as a PLC. The AWG was invented by Dragone by combining a dispersive array of waveguides with input and output “star couplers” on a planar lightwave circuit chip. The AWG can work both as a DWDM demultiplexer and as a DWDM multiplexer, as taught by Dragone in U.S. Pat. No. 5,002,350 (March 1991). Other dispersion devices such as echelle gratings can also be realized in PLC for the multiplexing/demultiplexing functions.
U.S. Pat. No. 7,027,684 issued Apr. 11, 2006 to Ducellier et al, and United States Patent Publication No. 2004/0252938 published Dec. 16, 2004 to Ducellier et al relate to single and multi-layer planar lightwave circuit (PLC) wavelength selective switches (WSS), respectively, which are illustrated in FIGS. 1 and 2. A single level device 1, illustrated in FIG. 1, includes a PLC 2 with an input AWG in the middle, and a plurality of output AWG's on either side of the input AWG. An input optical signal launched into the input AWG is dispersed into constituent wavelengths, which are directed at different angles through lensing 3 to an array of tiltable mirrors 4. The light is collimated in one direction, e.g. vertically, by a first cylindrical lens 5 adjacent to the PLC 2, while a cylindrical switching lens 6 focuses the output light in the horizontal direction onto the tiltable mirrors 4. Each wavelength channels falls onto a different one of the tiltable mirrors 4, which redirect the individual wavelength channels back through the lensing 3 to whichever output AWG is desired for recombination, and output an output port. For the single level device the tiltable mirrors 4 rotate about a single axis to redirect the wavelength channels within the dispersion plane, i.e. the plane of the PLC 2.
A two level device 11, illustrated in FIG. 2, includes a second PLC 12, similar to the PLC 2, superposed above the PLC 2 with a plurality of input or output AWG's and ports. A second cylindrical lens 15 is superposed above the first cylindrical lens 5 for focusing the beams of light onto the output AWG's provided on the second PLC 12. For the two-level device, tiltable mirrors 14 rotate about two perpendicular axes to redirect the wavelength channels within the dispersion plane (as above) and at an acute angle to the dispersion plane into a plane parallel to the dispersion plane, i.e. the plane of the PLC 12.
In the aforementioned Ducellier devices, the AWG's terminate in straight linear arrays at the edge of the chip, whereby without the curvature at the AWG outputs, the “foci” occur at infinity. Accordingly, an external, bulk-optic lens is required to function as more than simply a field lens, but as a full (spatially) Fourier transforming lens. Consequently, not only is the external lens required to be extremely well aligned, i.e. relatively expensive and extremely sensitive to misalignments, but the optical path is necessarily mostly in air.
An object of the present invention is to provide a tunable add/drop filter in a monolithic PLC.
A further object of the present invention is to overcome the shortcomings of the prior art by providing virtual pupils at the interface between the channel waveguides and the slab waveguide on the PLC for focusing each wavelength channel. A further lens system comprises a plurality of on-chip lenses on the PLC for transforming the focal plane of the spatially dispersive demultiplexer into a substantially flat plane at the edge of the PLC. On-chip lenses are realized as reflective surfaces within slab waveguiding regions having a surface curvature to provide optical power.